Okay, so if you'll want to start out by adding 300 to the 900 they want to raise. This is because they want to raise 900 dollars, and they've already spent 300 on prizes and stuff. So, they want to raise 1,200 in order to make back the money they've spent in addition to the 900 they want to make for the city park.
If they sell each ticket for $30, that means they have to sell 40 tickets to make the minimum amount of $1200.